Composition and process for phosphatizing metal



United States ?atent Oliice COMPOSITION AND PROCESS FOR PHOS- PHATIZINGMETAL Harry B. Copelin, Niagara Falls, N. Y., assignor to E. I. du Pontde Nemonrs and Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application July 1, 1955, Serial No. 519,670

20 Claims. (Cl. 148-615) This invention relates to a process forproducing phosphate coatings on metals. More particularly, it relates tothe production of phosphate coatings on steel or other ferrous surfacesin a nonaqueous medium. In one aspect, the invention is concerned withthe process for producing phosphate coatings. In another aspect, it isconcerned with a solution for producing the coatings.

Phosphate coatings are widely employed today to protect metals againstmild oxidation and to provide surfaces conditioned to receive paintfinishes. Generally such coatings are formed in somewhat complicatedprocesses involving aqueous solutions at several points. Thus the basemetal may sequentially be treated with phosphate applied in hot aqueoussolution and then passivated by chromic acid dissolved in water.Degreasing operations and hot or cold water rinses are utilized asnecessary. Such procedures yield phosphate layers which themselvesafford some protection against corrosion but which are probably of morevalue as bases for additional coatings of paint.

Various disadvantages appear, however, in presently used. phosphatizingmethods. The cumbersome and lengthy sequence of steps necessary isevident from the outline given. The use of aqueous solutions, however,gives rise to other but perhaps less obvious disadvantages. Water, ofcourse, tends to corrode iron if contact is prolonged for any length oftime. Workpieces must, therefore, be carefully dried after the rinses oraqueou solutions are applied. In addition, the presence of water greatlyincreases problems involved in removing grease from or degreasing themetal preliminary to the phosphatizing.

Degreasing can easily be carried out by contacting the metal surfacewith a degreasing solvent, particularly of the chlorinated hydrocarbontype. Trichlorethylene and perchlorethylene are typical degreasingsolvents but others, s'uch asmethylene and ethylene chlorides, may beused as well. These materials, however, are incompatible with water andremain isolated in a separate phase when contacted with the latter.Presence of the two phases may hasten the corrosion of the base metal.Separate phases may be formed either by carrying water into thechlorinated hydrocarbon of the degreaser or by carrying the greasesolvent into the aqueous phosphatizing bath. Thus it is sometimesnecessary to isolate the degreasing stage from stages involving aqueoussolutions by intermediate hot alkaline cleaning solutions and by rinsingand drying steps- As a result of the additional steps necessitated bythe incompatability of water and the chlorinated solvents, degreasing bymeans of these solvents is usually dispensed with in phosphatizing. Thesimplicity of the degreasing itself and the cleanliness obtainabletherewith are thus lost to the phosphatizing process.

A general object of the present invention is, consequently, provision ofa new and improved process for developing phosphate coatings on metalarticles. .A qfler object is provision of a phosphatizing method simplerthan those in customary use.

A specific object of the invention is provision of a phosphatizingprocess which utilizes no water or aqueous solutions. A further objectis provision of a process for phosphatizing metals directly compatiblewith degreasing operations employing chlorinated hydrocarbons.

An additional specific object is provision of a nonaqueous solution forproducing phosphate coatings on metals. A corollary object is theprovision of means for solubilizing phosphoric acid in specificnonaqueous media.

The above-mentioned and yet further objects are achieved in accordancewith this invention by a process consisting essentially of three steps.These steps, in sequence, are: 1) the ferrous, or other, article to bephosphatized is degreased with a chlorinated hydrocarbon in thecustomary manner; (2) the degreased article is dipped in a special,nonaqueous solution containing orthophosphoric acid; and (3) the dippedarticle is degreased to remove excess phosphate. Of these steps, thefirst is substantially conventional and the third can be accomplishedvery simply. The heart of the invention thus resides in the second step.

The phosphate coating produced in the nonaqueous bath is somewhatdifierent from that generally obtained from aqueous solutions. Thus itis lighter in color than the coatings heretofore produced, i. e., it isgray rather than black. X-ray studies also indicate that there may besome difierences in physical structure. The protection afiorded and thebase metal and the value of the phosphate layer as a substrate forpaints are, however, comparable to those obtained from aqueousphosphatizing baths.

In carrying out the process of the invention, preliminary degreasing isaccomplished in a conventional manner. The workpiece, dry and physicallycleaned of loose scale and dirt, is suspended for a short time in thehot vapors above boiling trichlorethylene or tetrachlorethylene.Alternatively, it may be dipped into the boiling solvent. In either casesulficient contact time should be allowed to permit the temperature ofthe workpiece to reach that of the solvent.

The degreased workpiece is transferred, while still hot, to thephosphatizing solution of this invention. The

phosphatizing solution consists primarily of a chlorinated,

and

wliere ,Rijrepres'ents anialkyl radical containing between" PatentedApr. 16, 1957 land 18 or more carbon atoms. The aliphatic chain may bebranched or unsaturated although straight-chain, saturated compounds arepreferred. Little solubilization can be achieved with compoundscontaining less than 6 carbon atoms because of the insolubility of theagents themselves. Best results are found where R contains between 8 and18 carbon atoms. Specific hydrogen phosphates that may be used includethose of the hexyl, octyl, decyl, dodecyl, stearyl, oleyl and likeradicals. When diaIkyl phosphates are utilized the two Rs within thesame molecule may be different.

The concentration of the solubilizing agent may vary to some extent. Aweight of agent equal to not less than about O.50-1% of the weight ofthe solvent should be'employed for best results. Up to about by weightcan be used to advantage when the agent is itself sufiiciently, soluble.Higher percentages of the alkyl phosphateshould not in general be addedsince they mayunfavorably affect the boiling point and other propertiesof the solvent. Some of the alkyl acid phosphates are themselves notvery soluble in chlorinated hydrocarbons. The quantity of these addedwill be controlled by their solubility since it is desired to avoid theappearance of a second phase in the solution.

' Chlorinated hydrocarbons containing 0.5l% or more.

by weight of an alkyl hydrogen phosphate will dissolve up to about 0.5%by weight of orthophosphoric acid. Between about 0.1 and 0.5% of theacid, when dissolved, will provide a good phosphate coating on ametallic Workpiece. Concentrations near the upper limit of solubilityare, however, preferred since they speed the formation of the coatingsand need replacement or replenishment less frequently.

It will be understood. that phosphoric acid must be maintained withinthe bath employed. Maintenance can be accomplished by regular periodicadditions of acid or by determination of the quantity present followedby addition of the requisite amount. The acid may also be replenished byoccasional slow additions to the. bath carried out in such a manner thatthe last small quantity added just fails to dissolve.

Thorough contact of workpiece with phosphatizing solution is readilyinsured by immersing it in the latter. The only important variablesinvolved in the immersion are the contact time and the temperature,atmospheric or ambient. pressure being assumed throughout.

Phosphate coatings will be formed at any temperature between about 0and: 120 C. when iron or steel articles are contacted with the solution.Formation is, however, very slow at the lower temperatures. At thehigher temperature, e. g., 100-1206 C.,. the alkyl phosphates begin todecompose. This decomposition is very appreciable at 120 C. For thesereasons, between. 80 and 100 C. is preferred. 1n some cases it isdesirable to maintain heated. vapor in the space above the solution. The

temperature chosen should then be near the boiling point.

of the solution. Trichlorethylen'e boils at around 86 C. and thus is anideal solvent for the purposes of the invention. Perchlorethylene,boiling at around 120 C., can also be used as the solvent but preferablynot at its boiling temperature. Mixtures of perchlorethylene andtrichlorethylene can be used to obtain boiling points intermediate thoseof the constituents.

At temperatures between 80-120 C. suitable coatings form in about 1-10minutes. At 60-80 C. up to about one-half hour may be required. Below 60C. the required contact time is too long for practical Under theconditons of pressure, temperature and concentration prevailing when thecoating step of the process is operated, small amounts of Water arecarried from the bath in the vapors produced. The orthophosphoric acidof commerce can thus be used with good results. Solutions even moredilute may also be employed but are less preferred.

In addition to the ingredients mentioned, i. e., the solvent, thesolubilizer and phosphoric acid, a fourth nonaqueouscomponent ispreferably included in the bath. This component is a polar compound suchas glacial acetic acid which serves, not to improve the phosphatizationdirectly, but to prolong the life of the bath. A concentration of about1% by weight is desirable. The reflux life of a bath containing noacetic acid is only 10-l2 hours. This can be prolonged to 200 hours bythe inclusion of the second acid. 7

Apparatus for carrying out the process of the inventio may besubstantially conventional and so simple that it need not beillustrated. If desired a degreaser containing two compartments forliquids and vapors, separated by a partition, and a sump for drainagecan be used. The two compartments and the zone above the sump should bein communication in the vapor phase. The entire apparatus can be open tothe atmosphere if it is provided with cooling coils at the top tomaintain a steady air-vapor interface.

The first compartment is maintained at a temperature above the boilingpoint of the solvent utilized and the second is maintained at thepreselected phosphatization temperature. A workpiece can thus becontinuously passed through the vapors of the first compartment, wherestandard vapor degreasing occurs, and then dipped below the surface ofthe liquid in the second compartment. After phosphatization the articleis withdrawn through the vapors over the bath. During this passage asecond degreasing takes place removing excess phosphate from thesurface. The workpiece is then drained for a short time over the sumpand removed from the apparatus substantially dry. The surface of themetal is immediately ready for further treatments, painting inparticular, as desired.

While the invention has been described primarily with reference to ironor alloys thereof, its application is not restricted thereto. Metallicsurfaces generally subject to phosphatizing can, in fact, be treated inthe chlorinated. hydrocarbon bath. Zinc and aluminum-zinc provideexamples of such metals in addition to cast iron and steel.

Advantages of this invention, generally paralleling the objects, areobvious from the foregoing. description. Some, however, may bementioned:

(1) A phosphatizing bath is provided which. elimihates the need forwater and is compatible with chlorinated degreasing solvents;

(2) The large number of steps required in most phosphatizing processesis' reduced to three; and

('3) Requirements for equipment, particularly in rinsing and dryingsteps, are minimized.

There follow some examples which illustrate the bath and process of thisinvention in operation. In these examples all. percentages are reportedin terms of weight. Pressures in all; vapor-phase. operations wereambient.

Example I To 270- g; of boiling trichlnrethylene; was. added 30 g; of acommercialoctyl hydrogen phosphate carrying dissolved 3 g. of. 85%.orthophosphoric acid. The commercial phosphate consisted. of about 40%of monooctyl dihydrogen phosphate, about 40% ofv dioctyl hydrogenphosphate and a remainder of trioctyl phosphate. The composite solutionwas boiled until water stopped. distilling over, additionalt'ri'chlorethylene being added to maintain the weight. 3 g; ofglacial'acetic acid was then added thereto.

e.,raaovo A'number of panels of 22 gauge steel were vapor degrea'sed andwiped to remove smut(carbonac'eous dc posits). They were immediatelydipped for two'minutes into the boiling solution described above andthen removed, cooled and again vapor degreased in trichlorethylene. Thedegreased panels were sprayed with two coats of a urea-formaldehydealkyd interior enamel, commonly used in refrigerators, and baked for 30minutesat 280 F. These panels are' referred to below as test panels. 7

Control panels of the same 22 gauge steel utilized above were preparedby degreasing the base metal, coating it directly with enamel and bakingthe enamel. The following tests were made to compare the two sets ofpanels:

(1) Blister test. After 1 month immersion in water at 100 F. test panelsshowed negligible blistering. The controls blistered badly, at least 50%of their total surface being blisters; I

(2) Bending test. A test panel was bent sharply. The paint film brokeinto a multiplicity of tiny cracks along the bend but showed no tendencyto peel. The enamel coat on a control panel subjected to the sametreatment broke with a single crack peeling away from the bend;

(3) Swelling test. A test panel was immersed for five minutes in boilingtric hlorethylene with essentially no damage to the enamel. Controlpanels were com pletely stripped of their coating in 10-30 seconds ofthis treatment;

(4) Filiform corrosion test. Test and control panels were scribedthrough their respective coatings and exposed for 1 month to theatmosphere over a saturated aqueous solution of potassium chloridecontaining a small amount of hydrochloric acid. 6080% less corrosion wasnoted in the test panels; and

(5) Salt spray test. Test and control panels were scribed and exposed at100 F. for 125-150 hours to a 20% salt-spray fog. i. e., a spray of anatomized 20% salt (NaCl) solution. At the end of the exposure theunder-rusting at the inscribed cuts was severe on the control butnegligible on the test panels.

It may be noted that the commercial alkyl phosphate mixture of neitherthis nor of succeeding examples contained very much free orthophosphoricacid. Tests showed that the addition of phosphoric acid was essential toobtain the coatings desired. Some other commercial samples labeled AlkylPhosphates may contain enough orthophosphoric acid to render theaddition unnecessary.

Example 2 Test panels formed as in Example 1 were compared withphosphatized panels prepared as follows: 22 gauge steel was degreased,washed, dipped in a conventional aqueous phosphatizing bath, washed,dipped in chromic acid, washed and finally painted with the alkyd enamelused on the test panels.

The coating on the test panels was found to be slightly harder than thaton the controls and somewhat more difiicult to cut with a knife.Corrosion resistance as shown by the filiform and salt spray tests was,however, slightly better on the control panels.

Example 3 Example 1 was substantially duplicated except that thephosphatizing solution, prepared by dissolving in trichlorethyle'ne 1%of the crude commercial octyl hydrogen phosphate containing about 5% ofcommercial phosphoric acid, was free from acetic acid. Comparison testswith the treated panels gave about the same results as in Example 1.

-- Example 5 Clean steel panels were immersed for one hour at 25 C. in atrichlorethylene solution containing 3% of oleyl dihydrogen phosphateand about 0.15% of orthophosphoric acid. The panels were then vapordegreased, painted and tested as in Example 1 with similar results.

Example 7 The tests of Example 1 were substantially repeated except thata commercial octyl hydrogen phosphate solution was used containing 60%of monooctyl dihydrogen phosphate, around 2030% dioctyl hydrogenphosphate and a remainder of trioctyl phosphate. The test panelsproduced were not quite as good as those of Example 1.

It will be obvious to those skilled in the art that variousmodifications can be made in the details described without departingfrom the spirit of the invention. Thus the phosphate solution can beapplied to the workpiece by spraying or by any other method equivalentto the dipping exemplified. Consequently I propose to be bound solely bythe appended claims.

Having described my invention, I claim:

1. A uniform, substantially water-free composition of matter suitablefor phosphatizing metals consisting essentially of a chlorohydrocarbondegreasing solvent containing between about 1 and 10% by weight of anacid alkyl phosphate in which the alkyl radical contains between about 6and 18 carbon atoms and an effective phosphatizing quantity oforthophosphoric acid.

2. The composition of claim 1 containing additionally between about 0.5and 1.5% by weight of glacial acetic acid.

3. A uniform, substantially water-free composition of matter suitablefor phosphatizing metals which comprises around 98% by weight of achlorohydrocarbon degreasing solvent, up to around 10% by weight of anacid alkyl phosphate in which the alkyl radical contains 6-18 carbonatoms and about 0.1-0.5% by weight of orthophosphoric acid.

4. The composition of claim 3 containing additionally between about 0.5and 1.5 by weight of glacial acetic acid.

5. The composition of claim 3 in which the chlorohydrocarbon degreasingsolvent is a member of the group consisting of trichlorethylene andperchlorethylene.

6. The composition of claim 3 in which the alkyl radical is a member ofthe group consisting of octyl and oleyl radicals.

7. A composition consisting essentially of trichlorethylene containing,dissolved, up to about 10% by weight of octyl acid phosphate and about0.l-0.5% by weight of orthophosphoric acid.

8. The composition of claim 7 containing additionally about 0.5-1.5% byweight of glacial acetic acid.

9. A composition consisting essentially of trichlorethylene containing,dissolved, up to about 10% by weight of oleyl acid phosphate and about0.1-0.5 by weight of orthophosphoric acid.

10. The composition of claim 9 containing additionally about 0.5-1.5% byweight of glacial acetic acid.

11. The process of phosphate-coating an article made at a m ta of the. cs cap ble of reacting with phosphoric acid to form a metal'phosphatewhich comprises imm rsin he article for etwee about 2 and 60. minme; a at mpera ure b twee about 25 and 120 n a comp ion cons sting essentiallyof a chlor ydrocarbon degreasing solvent containing about 1 and byweight of an acid alkyl phosphate in which the alkyl radical containsbetween about 6 and 18 carbon atoms and an efiective phosphatizingquantity of orthophosphoric acid.

12. The process of claim 11 in which the chlorohydrocarbon degreasingsolvent contains additionally between about 0.5 and 1.5% by weight ofglacial acetic acid.

13. The process of claim 11 in which the chlorohydrocarbondegreasingsolvent is a member of the group consisting of trichlorethylene andperchlorethylene.

14.. The process of claim 11 in which the alkyl radical is a member ofthe group consisting of octyl and oleyl radicals. 1 i

15 The process of phosphatizing a metal capable of being phosphatizedwhich comprises immersing the same for between about 2 and 60 minutes at-100"'-C. in a composition consisting essentially of trichlorethylenecontaining about 1 10% by weight of octyl acid phosphate and about0.1-0.5 by weight of orthophospho'ric acid.

16. The process of claim 15 in which the composition containsadditionally about 0.5-1.5% by weight of acetic acid.

17. The process of phosphatizing a metal capable of being phosphatizedwhich comprises immersing the 5391C. for between about 2- and minutes atlift- C. in a composition consisting essentially of trichlorethylenecontaining about by Weight of oleyl acid phos: phate and about 0. 1-0.5by weight of orthophosphot'ic acid.

18. The process of claim 17 in which the composition containsadditionally about 0.5.1.5 by weight of acetic acid. 1 7

19. The methodof solubilizing phosphoric acid in a chlorohydrocarbon,degreasing solvent which comprises supplying saidacid to said solventin the presence of be ween b utlan l' 10% y e gh of an acid alkylphosphate in which the alkyl radical contains 6-18 carbon atoms. ,20.The method of claim 19 in which the chlorohydroe carbon; degreasingsolvent is a member of the group consisting of trichlorethylene andperchlorethylene and the alkyl radical- ,a member of the groupconsisting of c ylan o y vra i a si I References Cited in the tile ofthis patent UNITED STATES PATENTS Great Britain Nov. 10, 1954

1. A UNIFORM, SUBSTANTIALLY WATER-FREE COMPOSITION OF MATTER SUITABLEFOR PHOSPHATIZING METALS CONSISTING ESSENTIALLY OF A CHLOROHYDROCARBONDEGREASING SOLVENT CONTAINING BETWEEN ABOUT 1 AND 10% BY WEIGHT OF ANACID ALKYL PHOSPHATE IN WHICH THE ALKYL RADICAL CONTAINS BETWEEN ABOUT 6AND 18 CARBON ATOMS AND AN EFFECTIVE PHOSPHATIZING QUANTITY OFORTHOPHOSPHORIC ACID.